This disclosure is directed to systems and methods for measuring an absolute charge to mass ratio of toner and a dielectric thickness of a photoconductor layer on a photoconductor in an image forming device, and for providing process control feedback, and diagnostic warnings or alerts when the absolute charge to mass ratio of toner in the image forming device or the dielectric thickness of a photoconductor layer falls outside a predetermined range of acceptable values to support image production.
In an electrophotographic image forming device, images are formed as electrostatic patterns on a surface. Typically, the surface consists of a cylinder or belt with a photoconductive material coated on an outer surface of the cylinder or belt. Electrostatic charge is deposited on the surface of the photoconductor layer. A light source is directed at the photoconductor layer surface in accordance with an image to be formed. Where the light source strikes the surface, the electrostatic charge on the surface is discharged in accordance with the light received at that point on the surface leaving an electrostatic charge pattern, or latent electrostatic image, that corresponds to at least a portion of the image to be formed. The photoconductor layer surface is then contacted with toner particles that carry a charge. Charged toner particles are attracted to, or repelled from, areas of the photoconductor layer that are electrostatically charged.
Toner collects on areas of the photoconductor layer to cancel the electrostatic potential on the surface of the photoconductor layer at any position until no further toner particles are attracted to that area. Therefore, the density of toner particles at any particular point on the surface of the photoconductor layer will be proportional to the charge on the surface of the photoconductor layer.
The toner is subsequently transferred from the photoconductor layer to an image receiving medium, such as, for example, paper or plastic substrates. The toner image is then fixed to the image receiving medium by, for example, heating the toner on the medium to fuse the toner particles to the medium with a combination of heat and pressure.
Because the amount of toner attracted to any part of the surface of the photoconductor layer is proportional to the surface charge on the photoconductor layer and the charge on the toner particles, the charge to mass ratio of the toner particles will affect the density of toner that adheres to the photoconductor layer. If the quantity of charge per unit mass of toner is high, the charge on the photoconductor layer surface will be balanced by only a small quantity of toner. On the other hand, if the charge per unit mass of toner is low, a large quantity of toner will be required to balance the charge on the photoconductor layer. Therefore, the contrast of any formed image depends on the charge to mass ratio of the toner particles. Accurate control of an absolute value of toner particle change to mass ratio is, therefore essential to maintaining high image quality (IQ) in formed images.
Every effort is made to produce toner with a consistent charge to mass ratio. This ratio, however, may vary for a number of reasons. These reasons may include changes in manufacturing processes of the toner particles, changes in humidity and temperature of the atmosphere that surround the toner particles both in storage and in use, exposure to light or other forms of radiation, contact of toner particles with surfaces that transfer charge to or from the toner particles, aging of the toner particles and processes that agitate, mix or move the toner particles.
Systems for measuring how a relative charge to mass ratio varies with time, and adjusting a device to account for variance and attempting to maintain a constant relative charge to mass ratio are known. For example, U.S. Pat. No. 5,212,522 to Knapp teaches a method for attempting to maintain a constant relative charge to mass value. This system discloses placing a known charge per unit area onto a photoconductor. The electrostatic charge potential of the photoconductor cylinder before and after developing the image with toner is measured. The difference between the two potentials is used to calculate the relative charge per unit area of the toner. The mass per unit area of toner may be determined by, for example, an optodensitometer that measures the reflected light from the toner on the photoconductor, the frequency shift of a piezo device that has toner particles attracted to its surface from a known photoconductor area, or by measuring the voltage discharge rate of a photoconductor exposed to light through the toner layer. Using these pieces of information, relative charge per unit mass of the toner is determined. The system then takes various steps to compensate for changes in the relative charge to mass ratio in order to maintain IQ. The method disclosed in this patent, however, has limitations. Among these limitations are that the surface voltage measurements before and after development with toner depend on the state of the photoconductor layer. It is known that photoconductor layer properties may creep or change with time due to, for example, aging and wear. Such changes in the physical properties of the photoconductive layer will have a significant impact on the value of the calculated relative charge to mass ratio determined by systems such as the one disclosed in Knapp. Further, the calculated relative charge to mass ratio will be adversely affected by properties of the toner particles themselves. For example, the diameter of toner particles will have a significant impact on calculated relative charge to mass ratios.
These and other deficiencies mean that the system disclosed in Knapp will be able to compensate for changes in relative charge to mass ratios of the toner, but unable to compensate for specific changes in the photoconductor layer and the toner particles themselves. Changes to the photoconductor layer and toner may also fool the systems into compensation for changes in relative charge to mass ratio that do not exist, or compensation by an incorrect amount. Because systems such as Knapp do not calculate absolute values for the charge to mass ratio of the toner particles, these systems cannot determine if the absolute charge to mass ratio has reached a point where compensation is no longer possible because the absolute charge to mass ratio is either so high or so low that the compensation will no longer be effective. This means that the image forming devices will form images with poor IQ with no indication to a user that there is any issue with the image forming device based on charge to mass ratio of the toner particle.